mystery. The CDC is scrambling to solve. How did a giant cruise ship, become a floating Petrie dish? Hundreds of passengers sick with a virus that moved from deck-t deck-to-like lightning. The cruise ship with a lot of miserable mentals onboard. Vacations ruined. ABC's linsey Davis. Reporter: The cruise line promises relaxation, fun and entertainment. Prepare to be truly amazed. Reporter: But instead for 3,000 passengers tonight high anxiety on the high seas. It just felt like one of the worst stomach bugs I have ever had in my life. Reporter: Passenger arnee Dodd is just 1 of the more than 600 people on the royal caribbean who've become violently ill. Everybody was so nervous they were either staying to their state rooms or a public place that wasn't too crowded. Reporter: Officials say this is likely a case of the highly contagious norovirus, which probably came onboard with a sick person or in contaminated food. If it arrived in, say, contaminated seafood, not only would everyone who ate the fish likely get sick, but the virus could spread to those who handled the fish and to anyone who touched the ice it was served on. Norovirus survives on surfaces and on hands. So, once it leaves the dining room, it can spread exponentially throughout the ship. The crew, sporting hawaiian shirts and sanitation masks, is scrubbing the ship from top to bottom. Why is the norovirus so tough to contain? It is able to survive in the environment for days, for weeks, sometimes for months. Reporter: CDC inspectors are onboard. In a statement, royal caribbean said, we think the right thing to do is to bring our guests home early. Linsey Davis, ABC news, New York.

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